Kabul, Afghanistan (CNN) -- The bodies of three American contractors who died in a plane crash in Afghanistan earlier this month have been recovered, NATO's International Security Assistance Force said Tuesday.

The C-12 plane went down in the mountains of Nuristan province October 13, but the military withheld information about it until the recovery operation was complete, ISAF said in a statement.

The crew were subcontractors working for Lockheed Martin, the company confirmed.

A spokesman named two of them as Jeff Lehner, a former Air Force member working for Sierra Nevada, and Randolph Bergquist, a former Marine working for Avenge. They are not releasing the third name at the family's request. Thomas Casey of Lockheed Martin confirmed the third victim was the co-pilot and also worked for Avenge.

The cause of the crash that killed them is not yet clear, but it is not thought to be enemy action, ISAF said.

A helicopter sent to recover the remains October 17 itself made "a hard landing" that required the crew to be rescued, ISAF said.

The helicopter was stripped of sensitive parts Wednesday and destroyed in place Sunday, ISAF said.